ABSTRACT: Eliciting broad and potent HIV-specific neutralizing antibody (Nab) responses represents a holy grail of HIV vaccine efforts. To date, our understanding of the potential of the human immune system to generate such responses comes from the study of a subset of HIV-infected individuals who generate broad Nab responses as a result of a dominant monoclonal response. In contrast to the singly infected individuals who have been the topic of these previous studies, there is limited data on people who are superinfected with HIV and acquire two distinct strains of the virus from different partners. We have shown that superinfected individuals have broad Nab responses that cannot be explained by a monoclonal response to the known HIV epitopes. Our preliminary detailed study of one individual suggests that superinfection leads to a polyclonal response that results from distinct responses to the two infecting viral strains. We hypothesize that the complex dynamic between viruses in superinfection may contribute to the unique polyclonal response. To address this hypothesis, we propose to take advantage of our well-characterized cohort of superinfection, which represents the largest collection of cases identified to date. Among these individuals, we have identified several with broadly neutralizing antibody responses that do not map to a single known epitope target. We propose to isolate monoclonal antibodies from these individuals and determine the basis for the breadth of their response and whether the response is polyclonal. We will define the epitopes of the antibodies and the role that the initial and superinfecting virus played in eliciting them. We will also determine the evolutionary process that led to these responses. These studies represent a unique opportunity to understand how to elicit a polyclonal response to HIV, which may present a higher barrier to escape and resistance than a monoclonal response.